feat(11-01): remove checkbox-disable restrictions for future tasks

- Remove isFuture guard in calendar_day_list.dart, pass canComplete: true always
- Remove isFuture check in task_row.dart, always enable Checkbox.onChanged
- calendar_task_row.dart unchanged (canComplete param already defaults to true)

lib/features/tasks/presentation/task_row.dart

@@ -42,7 +42,6 @@ class TaskRow extends ConsumerWidget {
       task.nextDueDate.day,
     );
     final isOverdue = dueDate.isBefore(today);
-    final isFuture = dueDate.isAfter(today);
 
     // Format relative due date in German
     final relativeDateText = formatRelativeDate(task.nextDueDate, now);
@@ -57,12 +56,10 @@ class TaskRow extends ConsumerWidget {
     return ListTile(
       leading: Checkbox(
         value: false, // Always unchecked -- completion is immediate + reschedule
-        onChanged: isFuture
-            ? null // Future tasks cannot be completed yet
-            : (_) {
-                // Mark done immediately (optimistic UI, no undo per user decision)
-                ref.read(taskActionsProvider.notifier).completeTask(task.id);
-              },
+        onChanged: (_) {
+          // Mark done immediately (optimistic UI, no undo per user decision)
+          ref.read(taskActionsProvider.notifier).completeTask(task.id);
+        },
       ),
       title: Text(
         task.name,